Stains or discoloration on your roof can be alarming to most homeowners, because they tend to think that this is a sign of trouble. But, did you know that not all discolorations are symptoms of serious roof damage? So let out that deep breath you’ve been holding and relax before you go investigating. Even though the discoloration is probably cosmetic, you might find yourself wanting to get it fixed. Nothing wrecks curb appeal quite like a stained or slimy roof.

We can usually find the cause of a discolored roof just by knowing the location and color. Usually, what we find is a relatively harmless algae colony that needs to be cleaned and treated.

The Usual Suspect: Green or Black Algae Stains

Across the entire eastern half of the country, roof shingles tend to grow a species of algae called Gloeocapsa Magma. It appears in streaks or patches of slime that can be anywhere from blue-green to dark black.

Roof algae rarely poses a threat to the health of the roof. Algae does not endanger your home or health the way mold can.

Cleaning involves oxygen bleach or a solution of trisodium phosphate applied by a low-pressure wash. For your safety, it’s definitely a good idea to have a roof cleaning service take care of this for you.

As for prevention, there’s some interesting chemistry involved. Copper and zinc are anathema to algae. We can add some small sheets of copper or zinc to high points on the roof above the discolored areas. When it rains, the water washes the metallic atoms over the roof below, killing and preventing algae without anyone having to lift another finger.

Other Causes of Roof Discoloration

Quite a few other issues can cause stains on a roof. The solution might involve anything from waterproofing the roof to pruning the trees in your yard. Here are some clues to follow:

  • Rust or chimney soot stains — Red or brown rust stains around chimneys is a common roof stain. The area can be cleaned and treated with oxalic acid to prevent rust.
  • White stains on slate roofs — Efflorescence, or salt stains from rain water, sometimes appear on slate shingles.
  • Trees & plant debris stains— Stain patches under trees or nearby may be due to seeds, berries, leaves, and other debris falling and decaying. Moss often grows in the shade of trees, too.
  • Water stains — If the discoloration seems to be damp, check the attic or ceiling below the roof stain and investigate for a leak.
  • Light spots on roof shingles – Dark colored asphalt shingles may appear lighter and patchy as they age. Look in the gutters for signs of asphalt granules that have washed off, and expect to need a replacement roof soon.